Broadly tunable lasers are indispensable components of optical test and measurements equipment, as they enable relatively straightforward characterization of spectral properties of various optical devices and spans of optical fiber, such as that employed in fiber-optic communication links. For example, measurements of polarization mode dispersion in deployed fiber and various types of fiber-optic devices typically involves optical frequency sweeping using a tunable laser. One problem that typically arises in such measurements is the determination of the exact laser frequency at various instances during the frequency sweep, or at least an accurate determination of changes in the optical frequency or, which is substantially the same, laser wavelength.
There are known in the art means to reference the optical frequency or wavelength of a tunable laser. For compact tunable lasers that can be used in portable devices, a Fabry-Perot (FP) filter is commonly used for frequency referencing and/or locking, as described, for example, in U.S. Pat. No. 4,914,662 to Mitsubishi. This approach is suitable for laser frequency locking to a static frequency, providing a good absolute frequency referencing for a static-frequency laser. Drawbacks of this approach for frequency referencing during a frequency sweep by a tunable lasers is the large number of indistinguishable resonance peaks provided by an FR resonator, and a relatively large spectral width of each peak; a typical Fabry-Perot would only provide one or two resonances across a 100 GHz frequency span. When used in a static mode, laser locking to an FP etalon typically provides about +/−2.5 GHz absolute wavelength accuracy. A better performance could potentially be obtained in terms of relative wavelength accuracy during a wavelength scan, provided that the FP transmission is finely analyzed with a software fitting routine, possibly improving the wavelength accuracy to about 1 GHz, which still may not be sufficient for many applications. Moreover, for fast laser scans, e.g. as fast as 1 millisecond (ms) duration or faster, it would be preferable to avoid software processing and use an analogue circuit such as a level-crossing scheme for ‘on the fly’ wavelength referencing.
An external wavemeter, based on the Michelson interferometer, is sometimes used for laboratory instrumentation. This is however a very complex, fragile, and slow equipment, which is unsuitable for field measurements and in the context of portable fiber-optic measurement tools.
A set of tunable filters could also be used for wavelength referencing, as described for example in U.S. Pat. No. 6,134,253 assigned to JDSU. This technique provides however only one wavelength calibration point, may not be sufficiently fast, and may require calibrations that maybe hard to perform in a portable device to a required accuracy.
Alternatively, light from the tunable laser to be referenced may be combined with light of a static, wavelength-stabilized laser, and a high speed photodiode can then be used to determine the resulting beat frequency. In the electrical domain, two relative calibration points could be obtained by monitoring the RF power at a given frequency, anywhere in a range 0-100 GHz with a suitably fast photodiode. This can be done using a narrow, high-frequency filter, and/or a local oscillator for down-conversion in frequency. More calibration points could be obtained using several filters or local oscillators. However, the complexity and cost of this solution could be too high.
U.S. Pat. No. 7,405,820 discloses an optical spectrum analyzing device that utilizes Brillouin scattering in a piece of optical fiber as a filtering mechanism for high-resolution spectral measurements of optical signals.
Brillouin scattering is caused by an interaction of acoustic waves and optical signals. Acoustic waves can cause variations of the density of a medium in which they travel. The density variations can effect optical gratings. Scattering of an electromagnetic wave by such acoustic gratings in an optical medium is known as “Brillouin scattering”. The frequency of the scattered electromagnetic wave in the Brillouin scattering is shifted with respect to that of the original electromagnetic wave due to the Doppler effect caused by the motion of the acoustic waves. Depending on the relative directions of the acoustic wave and the electromagnetic wave, the frequency of the scattered electromagnetic wave may be down-shifted to a lower frequency, which is known as “Stokes shift”, or a higher frequency, which is known as “anti-Stokes shift”.
Stimulated Brillouin scattering (SBS) is a nonlinear optical effect in which the optical pump wave with a higher optical frequency νp provides optical gain to the counter-propagating optical wave with a lower frequency νs. The effect occurs when the optical frequency difference between the two waves is equal to the so-called Brillouin frequency shift, which is in a typical optical fiber at 1550 nm is about 10.8 GHz. The gain profile of the Brillouin interaction in fibre is lorentzian; at 1550 nm, it has a linewidth of about 30 MHz.
In the device disclosed in U.S. Pat. No. 7,405,820, the signal under investigation is fed into an optical fiber together with a narrowband optical probe signal from a tunable high-coherence laser with a pre-determined wavelength. The probe signal propagates in the opposite direction to that of the analyzed signal, such that both signals interact inside the fiber owing to the Brillouin effect. The Brillouin scattering effect enables a selective, narrow-band optical amplification of a determined component of the optical spectrum of the analyzed signal. The wavelength accuracy of this set-up relies on the wavelength accuracy of the tunable probe laser. In practice, in order to provide high wavelength accuracy, conventional tunable lasers require external wavelength referencing means, such as some combination of a wavemeter, fine-etalon reference and a gas cell for providing an absolute reference point, which leads to a bulky system that is not easily portable.
An object of the present invention is to provide a relatively simple and economical apparatus and method for an accurate optical frequency referencing of a tunable laser that would improve upon at least some of the aforedescribed deficiencies of the prior art, and would be suitable for use in a portable test equipment.